1. Field of the Invention
The present invention relates to a roll-sheet feeding device that unwinds and feeds a roll sheet wound in a roll, and an image forming apparatus employing the roll-sheet feeding device.
2. Description of the Related Art
Conventionally, it is known that a roll-sheet feeding device is provided in an image forming apparatus such as a multifunction product including such functions as a copier, a printer, and a facsimile, or including at least two of the functions, and that a roll sheet is used therein as a transfer material. In this type of roll-sheet feeding device, the roll sheet is rotatably supported through a spool or the like, and when the roll sheet is to be fed, a trailing edge thereof is pulled by a feeding unit, the roll rotates, and the roll sheet is unwound. At the time of feeding the roll sheet, if the roll sheet inertially rotates due to feed acceleration and inertia force so that a portion more than that required for a fed portion is unwound, slack occurs in the roll sheet. This phenomenon depends on a paper feed speed and an outer diameter/mass of the roll, and the inertia force particularly increases and the slack thereby increases under such conditions that paper feed acceleration is high, an outer diameter of the roll sheet is large, a sheet width is wide, and density of the sheet is high. When the slack occurs, then nonuniform stripes and black stripes occur at a nip portion due to an abrupt change in tension force causing the sheet being fed to be skewed, which causes image quality to be degraded. Therefore, the roll-sheet feeding device is usually provided with a brake mechanism, which prevents the slack in the roll sheet.
As a conventional brake mechanism, Japanese Patent Application Laid-open No. 2006-315777 discloses a brake mechanism that stops rotation of a spool shaft through a torque limiter or a spring member for applying torque to rotation thereof in a rewinding direction of a roll sheet, provided in one end side of the spool shaft.
However, the brake mechanism disclosed in Japanese Patent Application Laid-open No. 2006-315777 has some problems that high torque load always occurs in a motor.
In a roll-sheet feeding device in this conventional technology, a bearing provided in the spool shaft is supported by a U-groove formed in a roll sheet holder allowing for operability upon replacement of the roll sheet. In this structure, when the roll sheet is pulled out, jumping force is biased to the roll sheet, so that the roll sheet almost jumps out of the U-groove. Particularly, in recent high-speed machines, because high jumping force is added to the roll sheet, any countermeasure against this problem is required.
It is considered to provide an appropriate locking unit, as the countermeasure, so that the spool shaft inserted into the U-groove does not jump out of the groove. However, if the locking unit is provided, then this requires a lock operation and an unlock operation upon replacement of the roll sheet, which causes the operability to be extremely worsened.
Moreover, in the roll-sheet feeding device, the roll sheet to be supported is getting lighter in weight by being used over time. At this time, because brake force of the brake mechanism is designed for a roll sheet that has nearly the largest diameter, back tension more than required is applied to the roll sheet when it becomes lighter in weight. Therefore, it is desired that the brake force of the brake mechanism can be also automatically adjusted according to a weight of the roll sheet.
However, in the roll-sheet feeding device according to Japanese Patent Application Laid-open No. 2006-315777, the brake force is always constant, and therefore, the brake force cannot be appropriately adjusted according to conditions.
A roll-sheet feeding device that can appropriately adjust the brake force according to conditions is disclosed in Japanese Patents No. 3454227 and 2951951. A technology disclosed in Japanese Patent No. 3454227 allows the brake force to be appropriately adjusted because the torque works by own weights of a flange and the roll sheet. However, when an outer diameter of the roll sheet is small i.e. its width is narrow under a high-speed condition, then the flange jumps, so that the rotation cannot be stopped by using friction between the roller and the flange, and a brake does not thereby work. A technology disclosed in Japanese Patent No. 2951951 allows the brake force to be optimally adjusted, however, an angle sensor and an electromagnetic brake are provided to control the brake force, which leads to an increase in cost.